This invention relates to collection optics for a high brightness discharge light source. More particularly, the invention relates to modification of known optical arrangements which use an elliptical or ellipsoidal reflector with a high brightness light source located at a first focal point of the reflector and an input end of a light receptive component such as a light guide or light conductor placed at a second focus of the reflector for transmitting light to a remote location. However, it will be understood that the invention has broader applications and may be advantageously employed in a number of other lighting environments and applications. Particular lighting systems require a high level of lumens for selected applications. For example, when using an optical fiber or light guide to carry the light to a remote location in the system, it is important to collect a maximum amount of the light emitted from the light source and input the light into the end of the light guide. The amount of light can be measured as number of lumens per unit area, i.e., lumen density, and the transmission optics plays an important role in maximizing the lumen density.
Typically, there is a tradeoff between lumen density and the cross-sectional dimension of the light guide. In general, the smaller the size of the light guide, the higher the lumen density. This higher density, however, comes at the expense of less collected lumens. Decreasing the size of the entrance aperture is also limited by the requirement that the transition optics must be sufficiently enlarged relative to the magnified image of the source in order to collect sufficient lumens. Therefore, it is generally preferred to maintain a slightly larger entrance aperture which, in turn, requires collection of a greater number of lumens from the light source to achieve the goal of increased lumen density.
An elliptical or truncated elliptical reflector is often used to direct light received from the discharge light source into the light guide. Unfortunately, any light emitted from the light source at a region adjacent to, but slightly spaced or offset from, the first focus is not directed into the light guide. Light from the offset region is imaged or focused at an area spaced from the input end of the light guide, and thus does not contribute to the light carried by the light guide. Lumen density in the light guide thus has no contribution from this additional light emitted from the light source.
By way of practical example, a core of an arc discharge light source is preferably located at the first focus of an ellipsoidal reflector, while a plume is also located inside the envelope but at a different region relative to the reflector. For example, the plume is located closer to the reflector if the arc is operated in a vertical direction and the open end of the ellipsoid is pointed downwardly. Sodium emission is primarily provided by the plume which is a region of slightly lower temperature than the arc core. In the core, the temperature is too high and completely ionizes the sodium. Thus, light emitted from the core does not result from sodium emission. Instead, convection currents within the envelope carry the sodium to regions of lower temperature and light is emitted in these lower temperature regions. Unfortunately, the regions of lower temperature are sufficiently spaced from the core, and thus spaced from the first focus of the reflector, so that this sodium light emission is not positioned at the second focus of the reflector. The sodium plume would, in fact, be focused well beyond the second focus of the reflector, missing entirely the entrance end of a small light guide. As a result of this arrangement, the light propagated through and emitted from the output end of the light guide is lacking in red color and has a green color since the wavelength spectrum of sodium emission is not transmitted through the light guide.
Moreover, different magnifications from different portions of the ellipse are obtained. This non-uniform magnification requires increasingly complex shapes and configurations to match the collector design and maximize the collection of light. Accordingly, there is a tradeoff between complexity and light collection that needs to be addressed.
A simple transition optical arrangement is desired between the light source and the input end of the light guide to maximize the lumen density, collect light from regions adjacent the first focus, and yet maintain simplicity and efficiency.